1. Field of the Invention
The present invention relates in general to an elastic bushing for flexibly connecting a shaft member and a tubular support member, primarily for damping or accommodating vibrations applied thereto in its axial direction. More particularly, the present invention is concerned with improvements in a fluid-filled elastic bushing which utilizes shearing of a viscous fluid having a high kinematic viscosity, for damping axial vibrations applied to the bushing.
2. Discussion of the Prior Art
There is known a vibration-damping elastic bushing of a type which is interposed between a shaft member and a tubular support member, for elastically connecting these two members, such that vibrations received in the axial direction of the bushing are particularly suitably damped or isolated. Some of this type of elastic bushings are required to exhibit excellent vibration damping capability for a wide range of frequency of the axially applied vibrations. These elastic bushings may be further required to provide a relatively low static spring constant for vibrations applied in the radial direction.
For example, a rear member mount used in a rear suspension of a semi-trailing arm type of a motor vehicle is required to provide a high damping effect for the axial vibrations of a frequency range of about 10-15 Hz, in order to reduce rattling of the rear suspension due to windup vibrations upon starting of the vehicle, and at the same time provide a high damping effect for the axial vibrations having higher frequencies of about 30-40 Hz, in order to restrain rattling of the rear suspension during running of the vehicle on a bumpy road surface. The same rear member mount is also required to provide a low static spring constant in its radial direction (which includes the running direction of the vehicle), for effectively dealing with the harshness shocks applied to the vehicle.
The known elastic bushing such as the rear member mount discussed above generally includes: an inner sleeve in which a shaft member to be flexibly connected to a tubular support member by the bushing is fixedly inserted; an outer sleeve which is fixedly fitted in the tubular support member; and an elastic body made of a rubber material, for elastically connecting the inner and outer sleeves for damping primarily the axially applied vibrations, based on elastic deformation of the elastic body. This known arrangement of the elastic bushing is structurally limited in its damping capability with respect to the vibrations applied in the axial direction. Thus, there has been a need of improving the elastic bushing of the type indicated above.
In the light of the above need associated with the elastic bushing employing the elastic body interposed between the inner and outer sleeves, there has been recently proposed a so-called fluid-filled elastic bushing which has a pair of fluid chambers formed in the elastic body such that the two fluid chambers are spaced apart from each other in the axial direction of the bushing. The fluid chambers are filled with a fluid having a relatively low viscosity, and are held in communication with a suitable orifice or orifices, or flow restrictor passage means, so that the axially received vibrations may be effectively damped, due to the existence of the fluid masses in the fluid chambers and restrictor passage means.
In the fluid-filled elastic bushing utilizing the low-viscosity fluid as indicated above, vibrations applied to the bushing in its axial direction cause the fluid to flow through the restrictor passage means between the two fluid chambers, whereby the vibrations may be effectively damped based on resonance of the masses of the low-viscosity fluid flowing through the restrictor passage means.
Since the elastic bushing with the fluid chambers filled with a low-viscosity fluid operates to damp the axial vibrations based on the resonance of the fluid masses, the damping capability of the bushing has a considerably high degree of dependence upon the frequency of the input vibrations. Namely, the fluid-filled elastic bushing in question cannot cover a wide range of frequency of the input axial vibrations. In other words, the frequency range over which the elastic bushing is capable of effectively damping the axial vibrations is considerably limited, as compared with the frequency range of the actually received vibrations that should be damped.
Described more specifically, if the elastic bushing utilizing a low-viscosity fluid is used as a rear member mount of a motor vehicle, and the restrictor passage means and other parts of the bushing are adapted so that the resonance frequency range of the fluid corresponds to a frequency range of about 10-15 Hz of the input vibrations, the rear member mount is highly capable of damping the vibrations of about 10-15 Hz frequencies, but is not capable of providing a sufficient damping effect for the vibrations of about 30-40 Hz. If the rear member mount is adapted to provide a high damping effect for the 30-40 Hz vibrations, on the other hand, the same rear member mount cannot satisfactorily deal with the 10-15 Hz vibrations. Thus, the known elastic bushing utilizing the low-viscosity fluid has a problem that should be solved.